This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7 119 from my application entitled ELECTRON GUN FOR COLOR CATHODE RAY TUBE earlier filed with the Korean Industrial Property Office on May 4, 2001 and there duly assigned Serial No. 24375/2001.
1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube (CRT), and more particularly, to an electron gun for a color CRT having improved electrodes arranged inline and forming at least one quadrupole lens.
2. Description of the Related Art
A typical electron gun for a color CRT is installed at a neck portion of the CRT to emit an electron beam. The performance of the color CRT depends on how the electron beam emitted from the electron gun lands on a fluorescent film. Accordingly, various types of electron guns have been developed to improve a focus feature and reduce aberration of an electron lens so that the electron beam emitted from the electron gun can accurately land on a fluorescent point of the fluorescent film.
In particular, in order to reduce the total length of a CRT, the diffraction angle of the CRT is increased so that the length of the electron gun is reduced. In this case, since the focusing length of an electron beam landing on the peripheral portion of the fluorescent film is relatively greater than that of an electron beam landing on the central portion of the fluorescent film, the focus feature of the electron beam at the peripheral portion of a screen deteriorates.
Also, as the deflection angle increases, the incident angle of an electron beam with respect to the fluorescent film decreases. Accordingly, the amount of distortion of an electron beam increases exponentially so that the diameter of the electron beam landing on the fluorescent film increases. A self-convergence type inline color CRT includes a deflection yoke forming a non-uniform magnetic field to deflect the electron beam emitted from the electron gun. The electron beams emitted from the electron gun are focused by means of a main lens formed in the electron beam, and converged throughout the entire area of a screen by the non-uniform magnetic field formed of a pincushion type horizontal deflection magnetic filed and a barrel type vertical deflection magnetic field by a deflection magnetic field.
The side electron beams of three electron beams passing through the non-uniform magnetic field and arranged inline is distorted relatively greater than the central electron beam. Thus, it is advantageous to the focus feature to make a correction force of the side electron beams greater than that of the central electron beam.
To adjust the profile and focus length of the electron beams, conventional electron guns adopt quadrupole lenses. The function of a quadrupole lens is as follows.
When electron beams emitted from a triode unit of an electron gun pass a pre-focus point and finally arrive at a quadrupole lens, a dynamic voltage applied to electrodes constituting the quadrupole lens is increased. Then, electrons forming the electron beams receive forces in directions in which electrons are divergent vertically and condensed horizontally, by an electric field forming the quadrupole lens. Thus, when the electron beams are deflected toward the peripheral portion of a screen, the distortion of the electron beam due to the deflective magnetic field, the incident angle of the electron beam, and the curvature of a screen surface is compensated.
However, during the increase of the dynamic voltage, a degree of focal length and astigmatism among the three inline electron beams has a difference of a degree of adjustment due to a difference in diameter of a large diameter lens, and the ratio of the horizontal and vertical lengths of the diameter.
An electron gun to correct astigmatism of an electron beam deflected toward the peripheral portion of a screen is disclosed in U.S. Pat. No. 4,701,677 for Color Cathode Ray Tube Apparatus by Ashizaki et al. and U.S. Pat. No. 4,814,670 for Cathode Ray Tube Apparatus Having Focusing Grids with Horizontally and Vertically Oblong Through Holes by Suzuki, et al.
In the electron gun disclosed in U.S. Pat. No. 4,814,670, to decrease astigmatism due to the deflection of an electron beam, a focus electrode is cut into two parts. A vertically elongated electron beam passing hole is formed in one of the cut electrode while a horizontally elongated electron beam passing hole is formed in the corresponding other cut electrode.
Another example of the conventional electron gun is disclosed in U.S. Pat. No. 5,027,043 for Electron Gun System with Dynamic Convergence Control. The electron gun is provided with means for converging the electron beam from a linear path. A quadrupole lens for correcting known a aberration by a self-convergence type deflection yoke is used as such means. The quadrupole lens means has a structure of applying different voltages to electrodes where a vertically elongated electron beam passing hole or a horizontally elongated electron beam passing hole are formed. Thus, the electron gun can correctly focus the three inline electron beams and correct a profile by vertical and horizontal deflective magnetic fields for the deflection of an electron beam.
As a color CRT has a wide deflection angle and is made flat, the difference between the focal lengths at the center of a screen and the peripheral portion thereof increases and the astigmatism due to the deflection yoke becomes greater. Therefore, the electron gun needs a strong astigmatism correction force and focal length correction force.
For the strong astigmatism correction force, since the electric potential difference between electrodes forming a quadruple lens must be wide and the focal length correction force must be large, a high voltage is needed at the peripheral portion of the screen. However, the high voltage causes problems in the reliability of a circuit and the withstand voltage between electrodes of an electron gun. Also, an electron beam is incident on the peripheral portion of a screen, the incident angle of the beam decreases in the horizontal direction and increases in the vertical direction by one of the functions of a quadrupole lens close to a main lens, that is, a function of making an electron beam condensed in the horizontal direction and diverged in the vertical direction. Thus, it is a problem that a horizontal diameter increases at the peripheral portion of a screen.
Meanwhile, astigmatism due to the deflection of the electron beam deteriorates as the deflection angle of the electron beam due to the deflection yoke and a convergence feature is deteriorated. To overcome the above problems, a color CRT with a quadrupole lens is disclosed in U.S. Pat. No. 6,051,919 for Color Cathode Ray Tube with Electrostatic Quadrupole Lens by Shirai et al.
In the above color CRT, the length of a plate of plate electrodes formed on the opposite surfaces of electrodes forming a quadrupole lens of an electron gun is formed such that the plate of the central electron beam is longer than that of the side electron beams. Thus, the central electron beam is strongly corrected to be focused for the correction of convergence and astigmatism at the peripheral portion of a screen.
However, in the CRT having the above quadrupole lens, as the deflection angle of the electron beam increases, a dynamic voltage applied to the electrodes forming the quadrupole lens increases at the peripheral portion of a screen. As the dynamic voltage increases, the focus feature is deteriorated at the peripheral portion of a screen due to the difference in the astigmatism correction effect at the peripheral portion of a screen between the central electron beam and the side electron beams which are arranged inline.
In particular, the above phenomenon becomes serious in an electron gun adopting a large diametric main lens. That is, when a dynamic voltage in synchronism with a deflection signal is applied to a large diametric electrode, the rate of a change of an electrostatic lens in vertical and horizontal directions of the central electron beam passing hole is greater than that of the side electron beam passing holes. The above phenomenon occurs since, in an area where the central electron beam passes, equipotential lines are gradually distributed in the horizontal direction compared to those in the vertical direction so that the effective diameter of the electrostatic lens is greater than that in the vertical direction and the rate of change in the vertical direction is greater than that in the horizontal direction during a change in strength of the main lens due to a change in the dynamic voltage.
However, since the side electron beams are located at the lateral side of the large diameter, during the change of a dynamic voltage, the effect of the change of equipotential lines in the horizontal direction is greater than the central portion. Accordingly, as the shape of the electron beam simultaneously changes in the horizontal direction, the rate of vertical elongation of an electron beam is less than that in the vertical direction. Thus, the dynamic voltage to deflect the side electron beams toward the peripheral portion of a screen should be higher than the dynamic voltage to deflect the central electron beam.
Another example of the conventional electron gun for color CRT is disclosed in U.S. Pat. No. 4,764,704 for Color Cathode-ray Tube Having a Three-lens Electron Gun by New et al. In the electron gun, a burring portion formed at two electrodes forming a quadrupole lens is cut into upper and lower portions and left and right portions and the cut portions are combined not to contact each other. Thus, a non-circularity of an electric field is corrected and the performance of the quadrupole lens according to the electron beam passing hole is maximized. However, it is not easy to form a lengthy burring portion of the electrodes forming the quadrupole lens of the electron gun. Also, reliability in processing according to the severance of the burring portion is low.
It is therefore an object of the present invention to provide an electron gun for a color CRT which can uniformly form an electron beam spot throughout the entire fluorescent film by improving a focus feature and correcting astigmatism due to a deflection yoke of three electron beams landing on the peripheral portion of the fluorescent film according to an increase of a deflection angle.
To achieve the above and other objects, there is provided an electron gun for a color CRT including a triode unit including a cathode, a control electrode, and a screen electrode for emitting an electron beam, and first and second focusing electrodes, installed coaxially with the triode unit, for forming at least one quadrupole lens, wherein protruding portions having flat surfaces corresponding to opposite surfaces of the first and second focusing electrodes are formed at least one of the opposite surfaces of the first and second focusing electrodes to change the profile of the electron beam by applying a dynamic focus voltage to the edge of electron beam passing holes formed in the opposite surfaces.
It is preferred in the present invention that vertically elongated electron beam passing holes are formed in the first focusing electrode and horizontally elongated electron beam passing holes are formed in the second focusing electrode, first and second protruding portions are formed at the horizontally opposite sides of each of the vertically elongated electron beam passing holes, and third and fourth protruding portions are formed at the vertically opposite sides of each of the horizontally elongated electron beam passing holes, and first and second flat surfaces are formed at the first and second protruding portions, respectively, and third and fourth flat surfaces are formed at the third and fourth protruding portions, respectively, thus forming a quadruple lens for changing the profile of an electron beam when a dynamic focus voltage is applied.
To further achieve the above and other objects, there is provided an electron gun for a color CRT including a triode unit having a cathode, a control electrode, and a screen electrode for emitting an electron beam, first and second auxiliary focusing electrodes sequentially installed coaxially with the triode unit, for forming auxiliary lenses, first and second focusing electrodes installed coaxially with the first and second electrodes, for forming a quadrupole lens, and a final acceleration electrode installed close to the second focusing electrode, for forming a main lens, wherein protruding portions having flat surfaces corresponding to opposite surfaces of the first and second focusing electrodes are formed at least one of the opposite surfaces of the first and second focusing electrodes to change the profile of the electron beam by applying a dynamic focus voltage to the edge of electron beam passing holes formed in the opposite surfaces.